The present invention relates to a process for determining the state of a battery in which there is a combination of a battery discharging operation by a specific current with a battery charging operation by a specific current, and in which a charging/discharging device is controlled in such a manner that the battery reaches a specific charging state, according to which the instantaneous terminal voltage U.sub.act is equal to a predeterminable reference value U.sub.ref2, and in which a signal is generated when the battery has reached the specific charging state, in which the instantaneous terminal voltage U.sub.act is equal to the predeterminable reference value U.sub.ref2.
A battery maintenance device is already known from German Patent Document DE 3,205,390 A1, and according to this document a battery is maintained by first charging it and then by discharging it with a preferably constant current to a predeterminable minimum value of the battery voltage and subsequently charging it again to the maximum value. For this, the battery capacity measured during the discharging operation is used as a criterion for the state of the battery to be maintained. This measured battery capacity is compared with the rated capacity, in order to reach a decision as to whether the battery is operative or not. This decision is indicated to the user of the battery maintenance device acoustically and/or visually. Furthermore, another process for determining the state of a battery is described, and in this process the battery is discharged down to the final discharge voltage, starting from the maximum terminal voltage obtainable. Subsequently, the battery is fed a charge quantity which corresponds to the total ampere-hour capacity of the battery. The state of the battery is derived by the measurement of suitable parameters.
Disadvantages arise in this known process, in that the battery is not tested and maintained with the discharge current that occurs during operation, with the result that a battery is still accepted as fault-free, even though, under some circumstances, it is already seen as no longer operative when loaded with higher currents.
The General Electric Company (in Germany: General Electric Plastics GmbH, Battery Department, D-609 Russelsheim, Eisenstr. 5) has published, under Publication Number GET-3148AD, a book entitled "Der Nickel Cadmium Akkumulator--Anwendungstechnisches Handbuch" [The nickel-cadmium accumulator--Practical manual]. From the second edition of this book it is known that the capacity of a battery decreases during its lifetime. It is also known from this book that the capacity of a battery decreases with an increasing discharge rate. Beyond these principles, this publication gives no indication as to how this knowledge can be used to advantage in the testing of batteries.
According to another known process for determining the state of a battery, German Patent Document DE-3,108,844 A1, a battery is discharged with a high discharge current. The terminal voltage of the battery is then measured during a time interval immediately after the discharge current has been cut off. The state of the battery is then deduced from the time trend of the terminal voltage in the regeneration phase of the battery.
An object of the present invention is to provide a process for testing a battery under conditions which correspond to the conditions of use of the battery, in order thereby to have the possibility of detecting a battery as inoperative in good time, and/or to test a battery which can no longer be charged to such an extent that it has the final charging voltage at its terminals.
This and other objects are achieved by the present invention which provides a process for determining the state of a battery comprising the steps of controlling a charging/discharging device to charge the battery to a specific charging state in which an instantaneous terminal voltage U.sub.act is equal to a predeterminable reference value U.sub.ref2, generating a first signal when the battery has reached the specific charging state, and controlling the charging/discharging device as a function of the first signal such that, during a specific time period t.sub.1, a specific current I.sub.1 flows, the direction of the specific current I.sub.1 corresponding to a discharging operation. A second signal is generated which controls the charging/discharging device such that a specific current I.sub.2, the direction of the specific current I.sub.2 corresponding to a charging operation, flows for a time period t.sub.2 until the battery is again in the specific charging state. The time period t.sub.2, during which the specific current I.sub.2 flows, is measured. The measured data is fed to an evaluation unit in which an order of magnitude of the conversion of electrical energy into heat energy is deduced from a deviation of the time t.sub.2 from the time t.sub.1.
One of the advantages of the present invention in relation to the known state of the art are that the points of the characteristic line characterizing specific states of the battery can be selected so that they are located in regions in which the characteristic line has a very steep trend. In other words, because of the relatively steep trend of the characteristic line at this point, the terminal voltage can be assigned to a point on the characteristic line relatively easily.
For carrying out the process according to the invention, the charge fed to and extracted from the battery has to be determined. This is obtained by means of an integration of the current. This integration can take place by measuring the current by sensing, the measured values of the current and of the sensing frequency basically producing the values which are used for an approximation of the integral by means of one of the known sum formulas.
In that part of the process according to the invention designated by process cycle I, it is possible to test batteries which, when being charged, no longer reach the full final charging voltage. It is expedient, when carrying out this part of the process according to the invention, to set the charging and discharging current at the order of magnitude of the relatively frequently occurring maximum discharge current of the battery under normal operating conditions. In this part of the process according to the invention, a balance of the charge quantity fed to the battery in relation to that extracted from the battery is prepared during a charging/discharging cycle of the battery. The heat energy consumed in the battery during such a cycle can be deduced from the difference between these charge quantities.
In that part of the process according to the invention designated by process cycle II, a battery in a stage I in which the battery is fully charged, that is to say at its terminals the instantaneous voltage U.sub.act is equal to the final charging voltage, is discharged with a discharge current considerably above its rated current. It is discharged until it has reached a state II, in which, at the terminals of the battery, the instantaneous voltage U.sub.act is equal to the final discharge voltage U.sub.SE. The so-called "dynamic internal resistance" is thereby detected, that is to say, during discharge with the high discharge current, the conversion of electrical energy into heat energy in the battery, clearly greater at high discharge currents, is taken into account. An indication of the capacity, taking into account the conditions of use of the battery is thereby made. In the process according to the invention, an appropriate order of magnitude for the discharge current can be in a range of approximately 50 times the rates current of the battery. For a different load of the battery in the operating state, other discharge currents can also be appropriate for carrying out the process according to the invention.
In certain embodiments for carrying out the process according to the invention, the discharge current is set at the order of magnitude of the maximum discharge currents relatively frequently occurring under normal operating conditions. At the same time, in carrying out the process according to the invention account is taken of the fact that a battery regenerates after a high load, that is to say its terminal voltage increases again when a discharge current is no longer flowing. This can be taken into account by, after a loading of the battery, discharging the battery again with a high current after a regeneration phase t.sub.delay, until it has reached the state II. The remaining capacity determined in this step is added to the capacity which was determined when the high current during the loading of the battery.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.